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Advice on minisplit placement

user-6861822 | Posted in Mechanicals on

I’m in the process of replacing baseboard resistance heat with mini splits, and could use some advice on placement. The house is wood frame, 2 x 6 wall, R-19 fiberglass and some R-23 rock wool in areas that have been redone, with 3″ of Comfortboard rock wool being added to exterior as siding is replaced. Roof eaves overhang 32″, so good shading. Outdoor 99% design temperature is 13 degrees; 1% is 89. Indoor design heating is 70 degrees, cooling is 75 degrees.
Climate zone 4A, 45 miles north west of Philadelphia.

The upper floor has sloped roof up to an 8′ level, insulated to R-45, with R-60 insulation above flat section in middle 8′ of rooms. A Manual J calculation (based on exterior being covered with Comfortboard sheathing) shows upper floor heat loss of 5179 btu, heat gain of 7220. First floor heat loss is 11,185, heat gain is 7220 btu.

First floor is over basement that is well-sealed at perimeter; walls are Superior precast, with foam insulation. I’m not sure of R-value – they currently advertise R-21, but the house is 24 years old and I doubt whether it is that well-insulated. Foam appears to be about 1 1/2 inches thick. There is R-19 fiberglass between floor joists supporting first floor.

I’ve attached rough plans of lower and upper floors. An HVAC contractor I’ve worked with has suggested a floor mount Fujitsu 9RLFFH unit at “x” position at right end of upper level and a floor mount Fujitsu 12RLFFH unit at “x” position at left end of lower level. We’d prefer floor units, for appearance reasons and to avoid problems that might result from mounting wall units too close to ceiling. Both bathrooms will probably have small baseboard resistance units.

Does this look like a workable plan? Are there better locations/sizing options for the indoor heads? Would it be worthwhile to increase size of first floor unit to 15RLFFH? Compressors for both will be located at rear of house, so line sets and drains will run to basement and out back.

Thanks for any advice.

Bruce

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The 9RLFFH is 2x oversized for the loads upstairs, but I'm not sure there's a great solution within Fujitsu's lineup. Mitsubishi's FH06NA would be a better fit for the load, and it's fine to mount high-wall units low. For a heat load of 5179 BTU/hr and a cooling load of 7220 BTU/hr the FH06 is about as good as it gets. It can deliver over 7500 BTU/hr at -4F, but can throttle back to have the minimum modulation of the 9RLFFH.

    http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submittal-sheets/9RLFFH.pdf

    https://nonul.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NA_ProductDataSheet.pdf

    Unfortunately Mitsubishi doesn't make a half-ton floor unit. The MFZ-KJ09NA/MUFZ-KJ09NAHZ isn't a much better load match than the 9RLFFH .

    There is no point to stepping up to the 15RLFFH (unless you're anticipating hell freezing over someday soon? :-) ) Your 99% outside design temp is probably still in double digits, or high single digits-F. If the load is 11K @ +10F you'd still be totally covered down to about -10F with the 12RLFF, since it can deliver 14,000 BTU/hr even @ -15F

    You might even consider dropping down the 9RLFFH for downstairs since it's somewhat better matched to the loads. It still delivers 14,000 BTU/hr @ +5F, and 11,000 BTU/hr @ -15F. That would be fully cover your load well into negative single-digits.

    The room by room loads would be useful for figuring out placement issues, but it's hard to heat a room at the far end of the house with a head at the opposite end, with hallways in between. A soffited mini-duct solution such as Fujitsu's xxRLFCD series would likely be more comfortable when it's actually cold outside.

    1. user-6861822 | | #2

      Hello Dana:
      Thanks for your response. I checked the Manual J again and see that I gave the wrong heat loss figures for the upstairs. Rather than 5179 Btu/hr, it's 4179. Room on left in plan is 1904 Btu/hr; on right is 2275. Checking at 47 degree design temperature, it looks like 769 Btu/hr on left and 919 on right. If I understand the submittal sheet information correctly, you're correct that the Fujitsu 9RLFFH would be way oversized under those conditions (minimum output of 3,100 Btu/hr), but you're also right that the Mitsubishi FH06NA should be able to work at the higher temperature. Just need to convince the contractor to install a Mitsubishi product; they seem to be partial to one or the other. The second level currently has a single 1250 watt baseboard unit in the lefthand room, which we rarely turn on. I'll check into the smaller Fujitsu for the downstairs.

      As to your thought about heat spreading throughout the space, one of the contractors we spoke with had looked into a mini-duct Fujitsu, but thought that there wasn't a suitable location for it. We keep doors open – right hand upstairs room is a studio space, so no great need for privacy. Also keep doors open downstairs. I had some concern that whichever room contained the head would either get overheated or overcooled, but we've been told that the temperatures tend to even out. We may lose some cool air to the downstairs via the stairway. How much do these things depend on the air movement generated by the fan. Do temperatures tend to reach an equilibrium throughout the space?

      Thanks again for taking the time to think about these things.

      Bruce

    2. user-6861822 | | #7

      Hella Dana:
      I just responded to Akos' message below, attaching floor plans with room by room heat loss/gain figures. You suggest, as does he, that it might work to use a mini-duct. I told him I'm not all that familiar with their workings, but if I understand correctly, it sounds like one in a central location, with relatively short, well-designed duct runs, might be used to condition the entire space. It looks like the Fujitsu 18RLFCD would work; the 12k Btu model might not have enough maximum cooling capability.

      On the floor plans I also marked one area in the upstairs hall with an "O". I wonder whether it would work to mount the Mitsubishi 6k wall unit you mentioned in that location. How much clear space is needed in front of the discharge? It seems this location is a bit more centralized, although it is also closer to the stairs, which may cause problems with loss of cool air to the lower level.

      I also marked two locations on the first floor as "O" and "O2". Wall mounts would work in either place and I wonder if moving the head to a more centralized location would make much difference in making it more comfortable. The one thing I like about using individual heads is that it appears they might be able to more closely meet the stated output under low temperatures. I notice that the slim duct unit says it will operate down to -5, although I assume this is not at full output. When/if it reaches that temperature, does it just stop operating?

      It also seems that the ducted models operate at a lower efficiency level, but I don't know what the real world implications are. If you know of any good articles covering this question I'd appreciate a link.

      Thanks for any information you can supply.

      Bruce

  2. Expert Member
    Akos | | #3

    Bruce,

    I would avoid a dedicated head in a bedroom, even the smallest head is too much airflow for comfort (setup I have at home, if I could go back, would definitely change it).

    For overall comfort and cost, I would vote for a single ducted unit in one of the upstairs closets. A duct through the floor and running to the two ends of the 1st floor and a small set of ducts upstairs to feed the bedrooms would not be a lot of work. The extra cost of ducting and bulkheads would be less than having two mini splits. If you keep the runs simple with only a few bends and no flex pipe, even the low static pressure ducted unit would work (I had no issues with a 0.2" w.g. rated ducted unit feeding 4 bedrooms and 1 bathroom).

    As for heat distribution, my place is about 1100sqft mostly open, if I just run the head in the living area (center of the house), the bedroom gets too hot for comfort in the summer.

    Good Luck.

    1. rk2 | | #4

      Akos - which indoor/outdoor combo do you have in your bedroom?

      I'm trying to decide on a setup, but don't see a good solution for low ambient systems:

      - Multi zone with one head per bedroom: Pro: one outdoor unit. Con: lowest output is way too high for much of the heating season (something like 6KBtu)

      - Multi zone with one ducted head for all bedrooms: Pro: one outdoor unit, can spread minimum output across rooms. Con: lousy individual temperature control.

      - Single indoor/outdoor for each bedroom. Pro: modulates down really well. Con: only seems to be available with wall mount, not other heads.

      1. Expert Member
        Akos | | #5

        I have an LG MultiF with a 24k 3 zone outdoor unit running two 9k and a 12k head indoors. They are heat pumps, but I only use them for cooling as the house has hydronic heat. 12k head center of living room/kitchen, 9k one bedroom, 9k for a future bedroom (currently part of living room).
        The unit is way over-sized, just running the 12k can keep the house comfortable except when the sun sets and I get a lot of solar gain on the west facing windows. Running the 9k head in the bedroom is possible, but I had to do some DIY modifications on the louvers to reduce the airflow over the bed. In hindsight I would have saved a lot of money and had a better system by not going with a multi split.

        With your load of ~17k heat and ~15k cooling load, a single ducted mini split for the whole house would be more then enough for the job.

        If you want separate controls for each floor, wall/floor mount unit in the middle of the main floor and a ducted unit upstairs would also work.

    2. user-6861822 | | #6

      Hello Akos:
      Thanks for your thoughts. I've attached revised floor plans, with the room by room heat loss/gain figures. I have a slight grasp of mini split technology, but must admit I'm not particularly conversant with ducted mini splits. Are you suggesting that I could install one ducted unit in an upstairs closet, sized to meet the loss/gain for the entire house, and run ductwork to the relevant spaces upstairs and downstairs – say, one vent in each upstairs bedroom, one in the living/dining area on the first floor and one somewhere toward the end of the hall on the first floor? I realize there are considerations with duct design, &c, but if this is feasible what are some of the pre and cons? Thanks for any further thoughts you may have.

      Bruce

      1. Expert Member
        Akos | | #8

        Bruce,

        That is it pretty much. You can treat a ducted unit the same a standard furnace and run the vents where you need them. If you are using a low pressure unit, you have to be careful about duct design especially takeoffs, bends and transitions (a round 90 deg adjustable elbow is equivalent to 45' of duct loss, loss from a couple of bends and a register boot add up quickly). There are some high pressure units which would be more forgiving. Some of the ducted units can also be mounted vertically.

        I wish I had decent pictures of the ducted setup I did for my father in law to share. The only issue I had, is the ducted units are quite large (reading the dimension on the data sheet is one thing, mounting it up is another) and doing nice swept bends for the branches took up a fair bit of space. Luckily it was all in the ceiling in the bathroom, you don't really even notice it once it is drywalled.

        As for pros and cons. Doing a single unit means you have to adjust registers in the summer/winter to balance loads. Finding an HVAC guys that will do proper sizing and nice ducting might be a challenge.

        Akos

        P.S.
        Bad ducting:
        https://static1.squarespace.com/static/51e577dce4b0ae9dfc23b1db/t/555c9e78e4b0391c3397ad33/1432133929072/

        Better:
        https://i.ytimg.com/vi/G3E9JBEZ5Zc/hqdefault.jpg

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